1. Field of the Invention
This invention is in the field of Synthetic Aperture Radar (SAR) imaging where missing pulses are encountered.
2. Description of the Related Art
Synthetic Aperture Radar (SAR) is used for ground mapping as well as target identification. The general principle behind SAR is to coherently combine the amplitude and phase information of radar returns from a continuous sequence of transmitted pulses. These transmitted pulses are from a relatively small antenna on a moving platform. As the platform moves, the information reflected from the sequence of pulses is coherently combined to arrive at a high resolution SAR image.
The plurality of sequential returns creating a SAR image generated by the transmitted pulses along a presumed known path of the platform make up an array. Theoretically, during the array, amplitude as well as phase information returned from each of the pulses, for each of many range bins, is preserved. The SAR image is formed from the coherent combination of the amplitude and phase of return(s) within each range bin, motion compensated for spatial displacement of the moving platform during the acquisition of the returns for the duration of the array.
The clarity of a SAR image is in many respects dependent on the quality of the motion compensation applied to each radar return prior to SAR image computation. Motion compensation shifts the phase of each radar sample (typically an I+jQ complex quantity derived from an analog to digital converter) in accordance with the motion in space of the moving platform with respect to a reference point. The SAR imaging process depends on the coherent, phase accurate summing of the sequence of ALL radar returns expected within an array. These principles are detailed by W. G. Carrara, R. S. Goodman and R. M. Majewski in Spotlight Synthetic Radar, Boston, Artech House, 1995.
In addition to the spotlight mode, SAR radar can also be operated in the search (swath or strip) map mode. Spotlight mode produces two dimensional images of limited size of a limited area by steering the antenna beam to the center of the map (image) center for the duration of a frame (or array). Conversely, search mode produces image strips of theoretically unlimited length by maintaining a fixed azimuth antenna orientation during a SAR-data collection period, or array. Unlike spotlight mode that typically produces images oriented in the range-azimuth direction, search mode produces images oriented in the along track and cross track direction. Uncompensated platform motion during SAR mode results in image smearing in the azimuth direction caused by pulse data that affects azimuth response.
Another difficulty presented during SAR mode comes from incomplete arrays. That is, a number of radar pulse returns forming the required sequence of returns in an array may be missing. The missing pulses may arise because of multi-mode operation wherein a different mode is entered during an array. The missing pulses within the partial array blur the resulting SAR image, obfuscating details thus rendering it of limited utility, necessitating restoration of the missing pulse returns.